Change default directory for the json file

[nicolaje]

Following discussion in #23 , we should target the TMP directory of the user instead of its Home directory by default for writing the .json file. On Windows, "." suffixed files are still visible, so it can give the impression that VIBes is messing with the user's directory for people unfamiliar with its internals.

• Identify environment variables for getting a good TMP directory on OSX, Linux, Windows (%TMP%)
• Change HOM to this TMP folder

[dvinc]

I think we can follow the approach of gettempdir() defined in Python module tempfile: https://docs.python.org/2/library/tempfile.html#tempfile.tempdir

Python searches a standard list of directories and sets tempdir to the first one which the calling user can create files in. The list is:

1. The directory named by the TMPDIR environment variable.
2. The directory named by the TEMP environment variable.
3. The directory named by the TMP environment variable.
4. A platform-specific location:
   • On RiscOS, the directory named by the Wimp$ScrapDir environment variable.
   • On Windows, the directories C:\TEMP, C:\TMP, \TEMP, and \TMP, in that order.
   • On all other platforms, the directories /tmp, /var/tmp, and /usr/tmp, in that order.
5. As a last resort, the current working directory.